Specimen analysis in an electron microscope

ABSTRACT

A specimen which is to be analyzed is initially introduced to an electron microscope housing at a location along the electron beam path between the objective lens and subsequent projection lenses in order to provide a smaller magnification relative to the magnification provided with the specimen in the normal object plane of the objective lens, but which provides greater mechanical facility in effecting the entry and removal of the specimen from the housing. While entry at this location results in less magnification of the specimen and image resolution, it is nonetheless suitable for a survey of the specimen in order to determine the adequacy of the specimen for analysis at the object plane.

United States Patent Buchanan SPECIMEN ANALYSIS IN AN 211 Appl. No.:66,787

[52] US. Cl ..250/49.5 A, 250/495 B [51] Int. Cl ..H0lj 37/26, GOln23/00 [58] Field of Search ..250/49.5 A, 49.5 B

[56] References Cited UNITED STATES PATENTS 6/1967 Akahori ..250/49.57/1952 Page ..250/49.5

OTHER PUBLICATIONS Introduction to Electron Microscopy by S.

Wischnitzer, Pergamon Press, New York, 1962, pages vacuum PUMP 51 Oct.3, 1972 Primary Examiner-Wllliam F. Lindquist Attorney-Edward R. Hyde,Jr.

[ ABSTRACT A specimen which is to be analyzed is initially introduced toan electron microscope housing at a location along the electron beampath between the objective lens and subsequent projection lenses inorder to provide a smaller magnification relative to the magnificationprovided with the specimen in the normal object plane of the objectivelens, but which provides greater mechanical facility in effecting theentry and removal of the specimen from the housing. While entry at thislocation results in less magnification of the specimen and imageresolution, it is nonetheless suitable for a survey of the specimen inorder to determine the adequacy of the specimen for analysis at theobject plane.

1 Claim, 3 Drawing Figures cation. In a transmissiontype of electronmicroscope,

for example, the electrons which impinge upon the specimen are partlyscattered and partly transmitted through the specimen. The transmittedelectrons are controlled by a number of electromagnetic lenses andsubsequently projected on a fluorescent screen thereby forming a greatlyenlarged image of a portion of the specimen for detailed viewing. Theimpinging electron beam is formed by means comprising a source ofelectrons and an electromagnetic condensing lens system while thoseelectrons transmitted through the sample are focused and projected atthe fluorescent screen by objective, intermediate, and projectionelectromagnetic lenses. These lens systems along with the beam source,the fluorescent screen and the specimen are mounted in an evacuatedenvironment of a housing for the microscope.

A specimen which is to be analyzed with an electron microscope isprepared, deposited in a receptacle, and positioned at an object planeof the lens system. The object plane is generally located near a gapexisting in the electromagnetic objective lens. The specimen receptacle,which is introduced to the housing through an airlock arrangement, mustbe carefully manipulated in order to position the specimen at the gap ofthe objective lens. It frequently occurs that after preparation of thespecimen, entry of the receptacle through the airlock, and carefulmanipulation of the receptacle for positioning the specimen in theobjective plane, the specimen is found upon initial examination to beunsuitable for an analytical determination. The specimen may beunsuitable because of improper preparation, dehydration in the evacuatedenvironment or other degradation, and the specimen must then be removedand replaced. This of course is undesirable since it is unproductive andtime consuming.

Accordingly, it is an object of this invention to provide a method andmeans for enhancing the analysis of a specimen in an electronmicroscope.

Another object of the invention is to provide a method and means forestablishing the suitability of a specimen for analysis in an electronmicroscope.

In accordance with the general features of this invention, a specimenwhich is to be analyzed is initially introduced to an electronmicroscope housing at a location along the electron beam path betweenthe objective lens and subsequent projection lens system for providing asmaller magnification relative to the magnification provided at theobjective plane, but which provides greater mechanical facility ineffecting the entry and removal of the specimen from the housing. Whileentry at this location results in less magnification of the specimen andinferior image resolution, it is nonetheless suitable for a survey ofthe specimen in order to determine the adequacy of the specimen foranalysis at the object plane. The adequacy of a specimen for moredetailed analysis is noted and the specimen is then removed for thislocation and introduced to the microscope at the object plane of theobjective lens. The operator is thus assured that the sample, which isnow positioned for detailed analysis, is suitable for detailed analysis.

An apparatus providing the general features of this invention includesmeans for positioning a specimen receptacle at a location along theaxial path of the electron beam at a point between the fluorescentscreen or other projection plane and the objective plane of theobjective lens. In a particular arrangement, transport means areprovided for simultaneously introducing a plurality of specimens intothe microscope housing at this location, for sequentially surveying allof the specimens and for simultaneously removing the survey specimensfrom the housing.

These and other objects and features of the invention will becomeapparent with reference to the following specifications and to thedrawings wherein:

FIG. 1 is a cross sectional view of an electron microscope constructedand operated in accordance with the features of the invention.

FIG. 2 is an elevation view, partly in section, of an embodiment of aspecimen survey transport means fabricated in accordance with theinvention; and

FIG. 3 is a plan view of the transport of FIG. 2. Referring now to FIG.1, an electron microscope is shown to include a cylindrical housing 10within which a relatively high vacuum is established. A vacuum isestablished in the upper sections of the microscope by a pumping system12 which is coupled to the housing through a conduit 14 while the lowerstages are evacuated through a conduit 18. A beam of electrons 20 isderived from a well-known source of electrons, which for simplificationin the drawings is represented by a rectangle 22 in FIG. 1. Theelectrons provided by this are accelerated centrally along the length ofthe cylindrical housing 10 toward a fluorescent screen 24. Themicroscope of FIG. 1 is illustrated to be of the transmission typewherein the electrons of beam 20 impinge upon and are partiallytransmitted through a sample 25 which is located in a receptacle 26.Those impinging electrons which penetrate the sample are acceleratedtoward the fluorescent screen 24 by a high voltage, not illustrated,where they excite the screen phosphors and provide a magnified image ofthe sample.

An electron optical system is provided in order to collimate the beam 20of electrons which impact the specimen and in order to focus and projectat the screen 24 those electrons which are transmitted through thespecimen. The electrons from source 22 are collimated into the beam 20by a first electromagnetic condensing lens comprising a core 30 offerromagnetic material and an electrical winding 32 positioned withinthe core 30 and having a plurality of turns. An electromagnetic fieldestablished by this lens is proportional to a current flowing in thewinding 32. Circuit means, not illustrated, are provided for varying thecurrent in this winding as well as the current in windings of otherelectromagnetic lenses, referred to hereinafter. A secondelectromagnetic condensing lens 34 is provided and is constructed in amanner similar to the first lens. These lenses are supported in thecylindrical housing 10 on plates 36 and 38 which are mounted to an innerwall of the cylindrical housing 10. Each of these plates includes acentrally located aperture through which the electron beam istransmitted.

Focusing and projection of the transmitted electrons of electron beam isaccomplished by an electromagnetic objective lens 40, an intermediatelens 42, and a projection lens 44. The objective lens 40 is mountedbetween plates 46 and 48 and extends between the inner walls of thehousing 10. The plate 46 includes a centrally located aperture while theplate 48 includes a slot extending diametrically therethrough forpositioning an aperture stop and, in accordance with the features ofthis invention, a specimen survey rod, discussed hereinafter. Aferromagnetic core 49 of the objective lens 40 includes a gap near anupper section thereof as viewed in FIG. 1. The object plane of the lensis located in the area of this gap. The specimen receptacle 26 which ispositioned on the plate 46 extends through the aperture in the plate 46and locates the specimen in the gap. For purposes of illustration inFIG. 1, the sample receptacle 26 is shown vertically displaced above theobject plane location. In operation, the receptacle 26 is loweredthrough the aperture in the plate 46 and a shoulder of this receptaclewill support the receptacle suspended from the plate 46 with thespecimen 25 located in the gap at the object plane.

An aperture stop, not illustrated, is in practice and during detailedanalysis of a specimen located at an intermediate focal point betweenthe objective lens 40 and the intermediate lens 42 for providing anelectron beam of desired diameter. The intermediate lens 42 ispositioned between a support plate 48 and a support plate 50. The lattermounting plate includes an aperture centrally located therein forpassage of the electron beam therethrough. The electromagneticprojection lens 44, which is generally similar in construction to theobjective and intermediate lenses and which includes a core offerromagnetic material and a winding positioned therein, is suspendedfrom the plate 50.

An air lock for introducing a specimen to the evacuated environment andfor removing the specimen without significantly interrupting theevacuation atmosphere is represented by the rectangle 52. The air lock52 includes an intermediate chamber operating at an intermediatepressure through which the specimen receptacle passes when introducingthe sample into the instrument. The further details of an air lock arewell known and for convenience in the presentation are not shown. Amechanical means, illustrated as the manipulating tongs 54, is providedfor locating the specimen receptacle 26 centrally within the microscopehousing 10, for lowering the receptacle into the gap of the objectivelens 40, and for removing the same when desired. An air lock 56 alongwith a means comprising tongs 58 is provided for positioning an aperturestop at the location between the objective and intermediate lenses. Asdiscussed hereinafter, the tongs 58 are also employed for manipulating asample holder at this location.

In operation, a sample is positioned in the receptacle 26 and isintroduced into the specimen stage of the microscope through the airlock 52. The specimen receptacle 26 locates the specimen in the gap ofthe objective lens at or near the object plane. An aperture stop ofdesired diameter, not illustrated, is introduced through air lock 56 andis located within the housing between the objective and intermediatelenses at the image plane of the objective lens. The lenses 30 and 34function to collimate the electron beam 20 and the electrons of thecollimated beam 20 which impinge upon the specimen at the image plane ofthe objective lens will extend through the aperture stop and will befocused by the intermediate lens 42. The electron beam is then projectedat the fluorescent screen 24 by the projector lens 44 causing a greatlymagnified image of the sample to be produced at this screen. Themagnified image can be viewed by an operator of the instrument through atransparent window 60 located in the housing wall.

As indicated hereinbefore, the introduction of a sample through the airlock into the housing and the manipulation of the specimen holder 26into the object plane can be relatively time consuming. The subsequentdiscovery that the specimen is defective or improperly prepared or hassuffered some degradation generally renders attempts at detailedanalysis ineffective. The steps involved in introducing the specimeninto the instrument have therefore been not only time consuming butinefficient. In accordance with the features of this invention, one ormore specimens are surveyed at a location along the electron beam pathbetweenthe object plane and the fluorescent screen 24. A survey as usedin these specifications and claims refers to viewing a specimengenerally in order to ascertain its characteristic of suitability forfurther detailed analysis. Although the magnification factor and imageresolution at a location alternate to the object plane of the objectivelenses will be substantially less than that provided at the objectplane, nonetheless the magnification is sufficient for surveying theprepared sample in order to establish the worthiness for detailedexamination at the object plane of the object lens. In the illustrationof FIG. 1, the aperture stop is removed and a survey transportarrangement is located at the image plane of the objective lens. Aplurality of specimens are positioned on a transport body which isreadily located in the instrument. The specimens are sequentiallypositioned in the beam path in order to project their image at thescreen 24.

FIG. 2 is an enlarged view, partly in section of the transport body 70for use with the sample survey technique of this invention. The body 70comprises a rod which has formed therein a plurality of longitudinallylocated apertures 72. These apertures are conically shaped. A specimen76 is deposited on a lower surface 77 of this pan, the surface 77 of thepan comprising an electron permeable material. A plurality of specimenpans are positioned on the rod 70. The apertures 72 are locatedequidistantly apart and an associated cove or depression 78 is providedfor each of the apertures 72. The coves 78 comprise a seat for a ball 80which engages the cove and functions as a detent mechanism. The rod 70extends through a guide groove 79 formed in the support plate 48 of themicroscope. A vertically extending screw 82 is adjustable forestablishing a desired compression of a spring 84. The spring 84provides a resilient force against which the ball 80 acts when the rodis pushed or forced in a longitudinal direction. The rod 70 is advancedby a force exacted thereon through extension rod 86. Each of the coves78 is located with respect to an associated aperture 72 for establishingthe positioning of a specimen of the associated aperture in the electronbeam path when the cove is detented and locked by the ball 80.

Each of the specimens-located in the pans 74 are surveyed at the lowermagnification in order to establish their suitability for furtherdetailed examination in the specimen stage. The survey of a plurality ofthe specimens is accomplished without removing other specimens from theinstrument while subsequent ones are being surveyed. Upon completion ofthe survey of all of the specimens at this survey station, the specimensare removed through the air lock 56, the aperture stop is replaced, andthose specimens found to be suitable for further examination can beindividually introduced to the specimen stage through air lock 52.

Thus a method and apparatus have been described which enhance theoperation of an electron microscope by assuring the operator of theinstrument that the specimen to be examined at the specimen stage isproperly prepared and suitable for analysis. The loss of time asexperienced with prior apparatus during the entry and removal of anunsuitable specimen at the object plane is therefore avoided andefficiency of analysis is greatly enhanced.

While the arrangement of FIG. 1 illustrates the survey station locatedin the electron beam path at the image plane of the objective lens, thesurvey station may equally well be located at the object plane of anylens in the microscope projection system.

While I have illustrated and described a particular embodiment of myinvention, it will be understood that various modifications may be madetherein without departing from the spirit of the invention and the scopeof the appended claims. What is claimed is: l. A method of electronmicroscope specimen analysis in an electron microscope includingelectromagnetic collimating, objective, intermediate, and projectionlenses comprising the steps of: generating an electron beam;accelerating said electron beam along a preestablished path forimpingement and penetration of a material positioned in said path;

positioning a material to be analyzed in said electron beam path at asurvey location positioned intermediate said objective lens and saidintermediate lens,

projecting the electrons which penetrate the material toward an electronresponsive means for providing a magnified image of said material; and,withdrawing said material from said survey location and positioning saidmaterial at a specimen location intermediate said collimating lens andsaid objective lens for detailed analysis of said material for providinga significantly greater magnified image of said material at saidelectron responsive means.

1. A method of electron microscope specimen analysis in an electronmicroscope including electromagnetic collimating, objective,intermediate, and projection lenses comprising the steps of: generatingan electron beam; accelerating said electron beam along a preestablishedpath for impingement and penetration of a material positioned in saidpath; positioning a material to be analyzed in said electron beam pathat a survey location positioned intermediate said objective lens andsaid intermediate lens, projecting the electrons which penetrate thematerial toward an electron responsive means for providing a magnifiedimage of said material; and, withdrawing said material from said surveylocation and positioning said material at a specimen locationintermediate said collimating lens and said objective lens for detailedanalysis of said material for providing a significantly greatermagnified image of said material at said electron responsive means.